using namespace RooFit;

void example_02()
{
    TFile *fin = new TFile("example_splusb.root");
    TNtupleD* nt = (TNtupleD *)fin->Get("nt");
    
    RooRealVar mass("mass","mass obs",0.,2.);
    
    RooRealVar mu("mu","signal mean",1.0);
    RooRealVar sigma("sigma","signal width",0.05);
    RooGaussian gaus("gaus","signal PDF",mass,mu,sigma);
    
    RooRealVar slope("slope","background slope",-0.3,-5.,5.);
    RooPolynomial linear("linear","background PDF",mass,RooArgSet(slope));
    
    RooRealVar ns("ns","ns",10,0.,1000.);
    RooRealVar nb("nb","nb",90,0.,1000.);
    RooAddPdf model("model","mass PDF",RooArgList(gaus,linear),RooArgList(ns,nb));
    
    RooRealVar slope_mu("slope_mu","slope mean",-0.3);
    RooRealVar slope_sigma("slope_sigma","slope sigma",0.03);
    RooGaussian cons("cons","constrained PDF",slope,slope_mu,slope_sigma);
    
    RooDataSet data("data","data",nt,RooArgSet(mass));
    
    model.fitTo(data,ExternalConstraints(cons));
    
    RooPlot *frame = mass.frame(Bins(25));
    data.plotOn(frame);
    model.plotOn(frame);
    frame->Draw();
}